1. Field of the Invention
The present invention relates to a piston for internal-combustion engines. More particularly, the present invention relates to a piston for in-cylinder injection-type internal-combustion engines, including diesel engines or direct injection-type gasoline engines.
2. Description of the Related Art
In pistons for internal-combustion engines, such as diesel engines or gasoline engines, there has been known a piston for internal-combustion engines, which has a low thermal-conductive member having low thermal conductivity at the top surface of the piston, which collides with the injected combustion gas, to thus inhibit the conduction of heat from the fuel collision part to the main body of the piston, thereby preventing the production of unburned hydrocarbons or smoke upon cold operations, such as when starting the internal-combustion engine.
For example, Japanese Unexamined Patent Publication No. 2000-186617 discloses a structure in which a low thermal-conductive member formed of a sintered titanium alloy material is mounted to the top portion (fuel collision part) of a piston made of an aluminum alloy. Since the sintered titanium alloy material has thermal conductivity corresponding to about 1/10 of the aluminum alloy used for the main body of the piston, the temperature of the wall surface of the combustion chamber of the piston is increased, and thus the atomization of fuel is promoted, resulting in improved combustion.
However, a rate of thermal expansion of the titanium alloy which is used for the low thermal-conductive member different from that of the aluminum alloy, and thus thermal fatigue failure undesirably occurs.